1. Field of the Invention
The present invention relates to a solid-state image pickup device.
2. Related Background Art
There are known various image pickup elements of the type in which signals are obtained by sequentially selecting horizontal pixel trains. Recently, there has been proposed an amplification type image pickup element of this type called a floating-gate-array (FGA) type.
A conventional FGA amplification type image pickup element will be described with reference to FIGS. 1 to 3.
FIG. 1 shows the fundamental structure of an FGA amplification type image pickup element (hereinafter called an FGA type element), and FIG. 2 is a timing chart showing the operation of driving FGA type elements.
Referring to FIG. 1, a plurality of photosensitive pixels 3 each comprised by a J-FET 1 and a capacitor 2 are disposed two dimensionally to constitute a photosensitive area (not shown). A V.sub.H pulse is applied to one horizontal line selected by vertical address lines to thereby read data on the selected line. V.sub.L pulses are applied to the remaining horizontal lines. In FIG. 2, a horizontal blanking signal is shown at (a), an address signal is shown at (b), a sense line bias signal is shown at (c), a reset pulse V.sub.H is shown at (d), a pulse V.sub.L is shown at (e), a clamp pulse .o slashed.CL is shown at (f), a sample hold pulse .o slashed.SH is shown at (g), and the drive timings of .o slashed.S.sub.1 and .o slashed.S.sub.2 are shown at (h).
The operation of the conventional FGA type element will be described while mainly referring to FIG. 2.
At FIG. 2(a) 101 during the horizontal blanking period, a sense line bias signal turns on at (c) 102 to enter a read standby state. At the same time, a read selection address signal indicates at (b) 103 a selection line (ADR=k). V.sub.L pulses for other horizontal lines not selected turn off at (e) 104 so as not to pickup data on the other horizontal lines. Next, a clamp pulse .o slashed.CL turns on at (f) 105, a sample hold pulse .o slashed.SH turns on at (g) 106, and sample hold capacitor C.sub.SH (refer to FIG. 1) is reset. Then, the reset pulse turns on at (d) 107 to discharge electric charges. Thereafter, the sample hold pulse turns on again at (g) 108 while holding its value in C.sub.SH. In order to conduct an electronic shutter operation, the address signal indicates another address line (ADR=L) at (b) 109. The electric charges on the selected line are discharged upon application of a reset pulse at (d) 110. After the horizontal blanking period at (a) 111, a shift register is driven at (h) 112 to read signals on one horizontal signal.
The electronic shutter speed control is performed depending upon the selection of address signals as shown in FIG. 4, i.e., upon a difference between the read address (l) and reset address (m).
In a video camera of an NTSC system using a conventional image pickup element having the above-described characteristics, the longest storage time period is generally 1/60 second. In such a case, driving methods shown in FIG. 3 have been used. Specifically, only address pulses among the pulses shown in FIG. 2 are changed to address pulses (2) as shown in FIG. 3. Immediately after reading data, the same pixel train is reset again.
Alternatively, only V.sub.H pulses are changed to V.sub.H (2) pulses. Only a reset operation for reading data is carried out, and a reset pulse for an electronic shutter is not generated.
Both the methods can set the storage time period to 1/60 second.
An electronic shutter performs an exposure control in combination with an exposure aperture. A shutter speed of 1/60 second is set mostly under a low illumination state among various light quantities.
It is well known that not only an FGA type element but also a phototransistor cell element has a problem of reset fluctuation. This reset fluctuation appears as fixed pattern noises (F.P.N) under a low illuminance state, thereby considerably degrading an image quality.
Recent various studies have shown that reset fluctuation depends on a reset time period, and that reset fluctuation is generally less for a longer reset time period.
However, the above-described reset timings are the same for any illumination state and therefore reset fluctuation has not been properly dealt with.